Since the introduction of multi-channel biochemical screening, there has been a four-fold increase in the incidence of primary hyperparathyroidism (PHPT), making it the third most common endocrine disorder. The clinical expression has changed, and most patients are now discovered accidentally, and lack either the complaints or the complications traditionally associated with this disease. Many different opinions have been expressed concerning the optimal management of such patients. However, the balance of risks and benefits of surgical intervention versus conservative follow-up can be determined only by a longterm multicenter randomized clinical trial. Thy hypothesis of interest in this trial is the null hypothesis that patients randomized to follow-up without surgery will have no more adverse effects than patients randomized to surgery. We propose to study a group of patients with mild asymptomatic PHPT and randomize them to surgery or no surgery. Eligible patients will have: (1) persistent hypercalcemia with mean values between 10.1 and 11.5 mg/dl without evidence of another cause; (2) absence of relevant symptoms or complications; (3) normal renal function; (4) normal bone density for age in forearm, spine, and hip; (5) absence of phalangeal subperiosteal resorption, vertebral compression fractures, or nephrolithiasis; and (6) no medical contraindication to surgery. One hundred such patients will be randomly assigned in blocks of 4-8 to standard surgical treatment or to conservative management, with stratification by sex. Both groups will be followed for a minimum of 36 and a maximum of 54 months, and compared with respect to rates of bone loss at each site, changes in renal function, incidence of fractures and nephrolithiasis, left ventricular function determined by Doppler Echocardiography, quality of life determined by SF-36 questionnaire, morbidity and mortality. Patients randomized to conservative management will be reevaluated every 6 months to ensure continued conformity to the entry and exclusion criteria. Additionally, this trial will serve as a feasibility study for a larger, longer term multicenter trial.